General thermoplastic synthetic fibers such as nylon or polyester fiber melt at about 250° C. However, heat-resistant high functional fibers such as aramid fiber, wholly aromatic polyester fiber and polyparaphenylene-benzobisoxazole fiber do not melt at 250° C., and a decomposition temperature of these fibers is about 500° C. A limited oxygen index of non-heat-resistant general fibers such as nylon or polyester fiber is about 20, and these fibers burn well in air. However, a limited oxygen index of heat-resistant high functional fibers such as those mentioned above is at least about 25, and these fibers may burn in air when they are brought close to a heat source of flame, but could not continue to burn if they are moved away from the flame. To that effect, a heat-resistant high functional fiber has excellent heat resistance and flame retardancy. For example, as a kind of heat-resistant high functional fiber, an aramid fiber is favorable to clothes for use at a high risk of exposure to flame and high temperature, for example, fireman's clothes, racer's clothes, steelworker's clothes, welder's clothes, and the like. Above all, a para-aramid fiber having advantages of heat resistance and high tenacity is much used for sportsman's clothes, working clothes and others that are required to have high tear strength and heat resistance. In addition, as it is hardly cut with edged tools, this fiber is also used for working gloves. On the other hand, a meta-aramid fiber is not only resistant to heat, but also has good weather resistance and chemical resistance, and it is used for fireman's clothes, heat-insulating filters, and electric insulators, and the like.
Heretofore, when a heat-resistant high functional fiber is formed into textile goods such as clothes, it is used merely in a form of non-crimped continuous filament yarn or spun yarn. However, when such non-crimped continuous filament yarn or spun yarn is woven or knitted into fabrics, and from them formed into clothes such as fireman's clothes, racer's clothes and working clothes, these resulting clothes are poorly elastic as the yarn itself is not elastic. As a result, when the clothes are worn, they are unsuitable to exercises and working activities. In particular, working gloves made of a non-crimped continuous filament yarn and a spun yarn are unsuitable to use in industrial fields of airplane and information instrument in which precision parts are handled, as they are unsuitable to exercises and working activities. Using the gloves mentioned hereinabove in those industrial fields often results in a lowering of productivity. Accordingly, an improvement of such a sort of disadvantages of heat-resistant textile goods that exhibit uncomfortable feeling when worn for a working activity is desired.
It is easy to produce a highly crimped filament yarn from general thermoplastic synthetic fibers such as nylon or polyester fiber by using heat-set. For example, known is a false-twisting method for crimping in which a thermoplastic synthetic fiber is twisted, heat-set and cooled. Also known is a stuffing box method for crimping in which a thermoplastic synthetic fiber is forcedly pushed into a rectangular space, and then heat-set.
On the other hand, it is impossible or very difficult to produce a crimped filament yarn of heat-resistant high functional fiber under the same process conditions and procedures as in the false-twisting method or the stuffing box method described above, since this heat-resistant high functional fiber is non-thermoplastic and therefore poorly heat-set. A crimping method which is suitable to a heat-resistant high functional fiber has not been established yet, so a heat-resistant high functional fiber has been used only in a form of non-crimped continuous filament yarn or spun yarn.
However, many studies and proposals have been made, relating to a heat-resistant high functional crimped yarn and to a method for crimping heat-resistant high functional fibers. Concretely, known are a method for producing a heat-resistant crimped fiber from heat-resistant fibers such as wholly aromatic polyamide fiber by selecting spinning conditions, without using a special crimping method and device (Japanese Patent Laid-Open No. 19818/1973), a non-heat stuffing box method in which an optical anisotropic dope such as para-wholly aromatic polyamide or the like is crimped in a stuffing box at room temperature and dried under a state of relaxation after performing a wet spinning method by dry-jet(Japanese Patent Laid-Open No. 114923/1978), a stuffing box method in which a high-elastic fiber such as a para-aramid fiber is crimped, and mixed with a low-elastic fiber (Japanese Patent Laid-Open No. 192839/1989), a method in which an aramid self-crimping filament yarn is produced by wet-and-dry spinning an optical anisotropic dope consisting of aramid and sulfuric acid under specific conditions (Japanese Patent Laid-Open No. 27117/1991), and a continuous process method in which an aramid fiber is false-twisted and crimped by use of a non-contact heater at a temperature not lower than that at which the fiber begins to decompose but lower than a decomposition point of the fibers (for a meta-aramid fiber, the temperature is at least 390° C. but lower than 460° C.), and thereafter subjected to heat treatment under relaxation (Japanese Patent Laid-Open No. 280120/1994). However, all of these known methods could still not solve outstanding technical problems which are how to realize easy process control, simplification of production lines, high productivity, and cost reduction. At present, therefore, no one has succeeded in industrial production of a heat-resistant crimped yarn exhibiting a good elongation percentage during stretching, wherein quality deterioration in a production process is reduced as much as possible.